help with mods for solid state on board reverb '70 carlsbro PA60R head

[bobster]

sorry to trouble you guys but i wonder if you could advise me how to improve the reverb sound in my circa 1970 amplifier.?
the reverb is a full length hammond tray and schematic shows it is driven by three BC207 and one BFY51 transistor. schematic here courtesy of john chambers site
http://www.chambonino.com/carlsbro/carl60par.html
it works but sounds very clanky so to speak and seems to have very little adjustment before it swamps the tone and makes things muddy. i have tried adjusting the trimpot but it doesnt really help . the reverb sounds almost like an analog delay with fast repeats and i was hoping it would sound more lush and subtle a la fender amp.
would any of you happen to know of any info or advice on modifications i can try or parts to replace / adjust?.
many thanks - rob

[Mark Hammer]

Years ago, I had a solid-state Gibson amp with a godawful reverb.  It too sounded muddy and excessive.  I found the cap limiting the bass content to the reverb pan and reduced its value, replacing it with a new cap.  Made a huge improvement, and the reverb was nice and bright and spacious sounding.

There are 3 electrolytic caps in the reverb circuit that might have become dried out and off spec: the 68uf leading to the pan, the 2.5uf receiving the output of the pan, and the 68uf gain-setting cap to ground for TR3.  You can probably replace all 3 of them with 50v-rated caps, changing the 68uf leading to the pan for 47uf.  Be mindful of orientation for the series caps (signal goes to "-" and comes from "+").

[bobster]

hi mark
thanks so much for the tips , i'll get on to that and see how it pans out [ groan ]   
bob

[runmikeyrun]

lol!  Classic reply!   

[Rob Strand]

I don't quite understand clanky and muddy.  Do you mean muddy when the reverb is set high?  Try reducing the 1000pF cap on the input to say 220pF and see if that puts things closer to what you want.   With this mod you will need to advance the reverb control quite a bit.  You can tweak the gain of the circuit later - first get the tone right then get the gain right.


ED: Actually what I have said is pretty much what Mark said.  The problem appears to be the drive circuit on you amp is voltage drive not current, which can sound muddy.  The smaller input cap simulates current drive.

[Johan]

+1 what Mark and Rob said...too much bass content will make the reverb clunky and muddy...if doing their mods isn't enough there is a very simple mod that works on all spring reverbs...simply take a flat nosed plier and randomly produce a kink in each spring. it will produce a node/reflection in the spring making the reverb more realistic.
the more expencive pans have these nodes built in by taking two different springs and joining them in the middle with a bead. but just putting a kink in lower end pans is an easy way of improving the quality of the sound..just don't overdo it, one kink in each spring is enough.
J

[bobster]

great stuff guys! thats a few good ideas to be going on with.
the connections to the tank are quite oxidised so i'll give everything a clean.
the springs are already joined in the middle like this but i may try a little kink anyway cos im a kinda kinky guy.........

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[bobster]

hey ,
well i found that the springs had some sticky gunge on them and when that was cleaned off it returned to a much nicer decay without a horrible resonance which had been creeping in.
the sticky stuff i think was vaseline that had somehow got on the springs after i had put a dab on the phone connectors to the tank [ when i drove motorbikes i used to put vaseline on the electrical connectors and it helped ]
D'OH !!

sometimes i feel a right tit ! [ sometimes i feel a left one too but dats anudder story ]